1. Agrobacterium mediated gene transfer

Agrobacterium tumefaciens and Agrobacterium rhizogenes are common gram-negative soil borne bacteria causing induction of ‘crown gall' and ‘hairy root' diseases. These bacteria naturally insert their genes into the genome of higher plants. The studies on crown gall formation revealed that the virulent strains of bacteria introduce a part of their genetic material into the infected cells where it gets integrated randomly with the genetic material of the host cell. The bacterial genes are able to replicate along with the plant genome and uses the machinery of plants to express their genes in terms of the synthesis of a special class of compounds, called opines, which the bacterium uses as nutrients for its growth but are useless to the host cells. In the process, Agrobacterium causes plant tumors (gall formation) commonly seen near the junction of the root and the stem and is called ‘crown gall disease'. A. tumefaciens attracted to the wound site via chemotaxis, in response to chemicals (sugars and phenolic molecules) released from the damaged plant cells. The disease afflicts a great range of dicotyledonous plants, which constitute one of the major groups of flowering plants. Tumorous plant cells were found to contain DNA of bacterial origin integrated in their genome. Furthermore, the transferred DNA (named T-DNA ) was originally part of a small molecule of DNA located outside the chromosome of the bacterium. This DNA molecule was called Ti ( tumor-inducing ) plasmid.

 

2. Plant gene structure

Plant ribosomal RNA genes and a number of other structural genes from a variety of species have now been analyzed in considerable detail. In common with many animal genes, some plant gene sequences have been found to have their coding sequences interrupted by introns or intervening sequences. These introns are transcribed but not represented in mature mRNA and hence, are not translated. No introns have been found in rRNA genes but they have been demonstrated in a number of other plant structural genes. A typical plant gene is shown in Figure 25.1.